Part I of this article “SPF Overview” covers publishing SPF on your name server: linuxjournal.com/article/7327

libspf: libspf.org

Milter: www.milter.org

SPF Download Page: spf.pobox.com/downloads.html

SPF wizard: spf.pobox.com/wizard.html

SRS: spf.pobox.com/srs.html